Method for the devulcanization of sulphur vulcanized elastomers

Abstract

Method for the devulcanization of vulcanized elastomers, comprising the following steps: A. preparing an aqueous mixture of vulcanized elastomers and a devulcanizing composition obtained by: i) mixing urea and at least one dicarboxylic acid in an aqueous solvent, for a time comprised between 3 and 8 hours, and at a temperature above 40 °C, to obtain an aqueous mixture comprising a devulcanizing ionic compound; and ii) adding a peptizing agent to said aqueous mixture coming from step i), to obtain the devulcanizing composition. B. carrying out a devulcanization reaction, by subjecting the aqueous mixture coming from step A to a state of stress, to obtain a devulcanized product.

Description

[0001] TITLE: “ Method for the devulcanization of sulphur vulcanized elastomers

[0002] DESCRIPTION

[0003] FIELD OF THE INVENTION

[0004] The technical field to which the invention belongs is that of methods for the devulcanization of sulphur vulcanized elastomers.

[0005] STATE OF THE ART

[0006] In the field of rubber, polymers are almost never used pure, except in rare cases. They are generally mixed with additives and accelerators of different nature, selected according to the final characteristics of the product to be obtained and the optimal processing conditions to obtain it. These mixtures of polymers and additives of various kinds are commonly referred to as mixtures.

[0007] Before the vulcanization process, the polymer chains present in a mixture do not have constraints, and are freely distributed in the mixture itself, which is therefore in the plastic state.

[0008] Vulcanization is a process for processing rubber, which is chemically bonded to sulphur through heating. In particular, sulphur allows the formation of bridging (or crosslinking) bonds, which bind the polymer chains of the rubber together.

[0009] This process makes it possible to obtain an elastic material, resistant to abrasions and tensile forces, intended to be used in production processes of different products and consumer goods.

[0010] From a chemical point of view, rubber vulcanization results in a modification of the molecular conformation of the polymer that composes it. In this regard, several studies have highlighted the formation of thioether bonds (R-S-R), both at the intermolecular level and, secondarily, at the intramolecular level, as a consequence of folding of the polymer chains. In particular, the addition of sulphur causes the loss of two hydrogen atoms, which lead to the formation of H2S; as a consequence of that, oxides such as ZnO favour vulcanization, fixing the sulphide.

[0011] These changes result in modifications in the rheological, chemi cal -physical and / or mechanical properties of the rubber. In particular, it was highlighted that the resistance to degradation with wear, a phenomenon that is accelerated in the presence of environmental agents, such as atmospheric oxygen, is substantially attributable to the bond of sulphur to the sites vulnerable to oxygen.

[0012] A spatial network of sulphur vulcanized elastomers usually has three types of chemical bonds: carbon-carbon (C-C), sulphur-carbon (S-C), and sulphur-sulphur (S-S).

[0013] Classification of elastomers

[0014] For the classification of the various elastomers commonly used in the rubber industry, reference is made to the standard DIN / ISO 1629 (derived from the earlier ASTM D 1418-79). Based on this classification, the group to which the polymer belongs is identified by the last letter of its identification code, while the remaining part of the code uniquely defines the elastomer as well as providing some more specific information.

[0015] The DIN / ISO 1629 classification provides for the synthetic rubbers to be divided into 5 groups:

[0016] • M group: polymers containing saturated polymer chains of polyethylene. This group includes the polymers known and identified with the following acronyms: EPDM, ACM, CSM, FEPM, FFPM;

[0017] • O group: polymers containing oxygen atoms. This group includes the polymers known and identified with the following acronyms: CO, ECO;

[0018] • Q group: polymers containing oxygen and silicon atoms. This group includes the polymers known and identified with the following acronyms: FMQ, MQ, PMQ, PVMQ, VMQ;

[0019] • R group: polymers containing unsaturated carbon. This group includes the polymers known and identified with the following acronyms: BUR, BR, HR, CR, HR, IR, NBR, SBR; • U group: polymers containing carbon, oxygen and nitrogen. This group includes the polymers known and identified with the following acronyms: AU, EU.

[0020] Devulcanization

[0021] Devulcanization is the process by which, by breaking the crosslinking bonds between the chains, a vulcanized rubber is transformed again into a plastomeric material, consequently making it reprocessable and recyclable.

[0022] In summary, through devulcanization, an attempt is made to return the vulcanized rubber from the elastic to the plastic state.

[0023] Below are several known methods for the devulcanization of sulphur vulcanized elastomers.

[0024] Mechanical processes

[0025] US5883140 A discloses a rubber regeneration process that consists of subjecting a mass consisting substantially of finely divided vulcanized rubber to high intensity impact forces, in a special mechanical equipment. This process allows to obtain a mass of re-vulcanizable rubber, with a tensile strength of at least 35% with respect to that of a similarly polymerized virgin rubber compound.

[0026] US5731358 A concerns a process for the production of high quality devulcanized rubber, starting from vulcanized waste and waste rubber products. In particular, said process comprises at least two stages, wherein: in a first stage a polymeric additive is mixed with the vulcanized waste rubber and the resulting mixture is cooled; and in a second stage unvulcanized virgin rubber is added to the mixture resulting from the first stage.

[0027] Microwave processes

[0028] US4104205 A discloses a devulcanization method of rubber by using microwave energy, which allows to produce a material that can be recompacted and re-vulcanized into a new vulcanized rubber-based article. This process is carried out by applying microwave energy to vulcanized rubber waste, at controlled doses, dosing rates and temperatures.

[0029] Ultrasonic processes

[0030] US5258413 A concerns a continuous ultrasonic method for breaking carbon-sulphur (C-S), sulphur-sulphur (S-S) and possibly carbon-carbon (C-C) bonds in vulcanized elastomeric particles.

[0031] Biotechnological processes

[0032] US5677354 A concerns a process for regenerating vulcanized or semi-vulcanized rubber, which consists in subjecting a mass of rubber to be regenerated to a solvent capable of swelling it, in passing the swollen rubber mass through restriction means provided with small openings, such as for example a perforated plate, in which devulcanizing agents are present in at least one of the restriction means and in the swollen rubber mass.

[0033] US5798394 A discloses a process for devulcanizing and functionalizing a rubber vulcanizate by desulphurization, which involves suspending a crumb of the rubber vulcanizate in a solvent that preferably swells the rubber vulcanizate before or during the devulcanization reaction and the addition of an alkali metal, such as sodium, to the formed suspension. The alkali metal performs the function of splitting the mono-, di- and polysulphuric crosslinks in the vulcanized rubber.

[0034] US5891926 A concerns a process for devulcanizing vulcanized rubber by heating at a temperature of at least about 150 °C and at a pressure of at least 3.4- 106Pa, in the presence of 2-butanol.

[0035] Processes employing a solid-based devulcanizer

[0036] EP0690091 Al discloses a solid-state devulcanizer and the relative process for devulcanizing elastomeric material polymerized with elementary sulphur, which comprises treating the polymerized elastomeric materials with one or more rubber accelerators and one or more activators, capable of initiating the proton exchange at temperatures below 70 °C and, therefore, of opening or breaking the bonds of the vulcanized network of the elastomeric material. EP1242520 B 1 discloses a modifier for the devulcanization of polymerized elastomers and, in particular, of vulcanized rubber, containing a first chemical substance, capable of dissociating and leading to the formation of an organic cation and an amine, and a second substance, capable of promoting the dissociation of the first substance, thanks to the presence of an acceptor functional group of said amine.

[0037] Problems of the background art

[0038] The methods for devulcanization known in the state of the art are usually based on techniques that exploit high temperature heating of the vulcanized elastomers. These processes are poorly selective on the breaking of crosslinking bonds (C-C, C-S and S- S), allowing to break even the strongest carbon-carbon bonds of the polymer.

[0039] However, the breaking of the C-C bonds generally involves a partial loss of the physical and mechanical properties of the devulcanizate with respect to the fresh starting product, i.e. the unvulcanized elastomers. As a result, these processes, in addition to requiring high energy consumption, are particularly invasive.

[0040] Furthermore, the processes involving the treatment of the compounds at temperatures of at least about 300 °C also lead to the generation of unpleasant odours.

[0041] Moreover, the devulcanization processes known in the state of the art, based on biotechnological reactions, such as for example those described in US5677354 A, US5798394 A, and US5891926 A, generally require the use of complex equipment, large spaces, long operating times, and rather expensive costs.

[0042] The disadvantages linked to the processes employing solid-based devulcanizers, in particular that described in EP0690091 Al, include: i) high costs; ii) short shelf-life, since, at the end of the process, the free sulphur in the mixture promotes a slow process of re-vulcanization of the devulcanized product; iii) poor penetration of the vulcanizate by protons, which act only in the surface areas and not in depth. To overcome this problem, it is necessary to pulverize the vulcanized material at least up to a particle diameter of 0.4 mm, with consequent increased costs and process times; iv) the treatment with protons does not show a good selectivity of action on the different crosslinking bonds of the devulcanizate, sometimes even breaking the C-C bonds, with consequent loss of the physical / mechanical properties of the product, such as tensile strength and percentage elongation at break.

[0043] With regard to patent EP 1242520 Bl, the process described therein has the following limits:

[0044] • it is not selective for saturated and unsaturated elastomers, being more effective on the elastomers belonging to the R group, according to the DIN7ISO 1629 classification, but less on other groups of the same classification;

[0045] • it has as a side effect the significant increase in the viscosity of the product, compared to the initial mixture, which makes the devulcanizate obtained more difficult to disperse in the mixture for subsequent reuses;

[0046] • it works with pressure values and with a friction coefficient, applied on elastomers in powder form, such that the additive-polymer aggregates are not broken down effectively, especially when the polymers present are saturated. The consequence thereof is the presence of dots on the surface of articles produced using the devulcanizate obtained by this process;

[0047] • it does not prevent recombination of the molecular chains of the rubber, just released by cross-linking with sulphur.

[0048] SUMMARY OF THE INVENTION

[0049] In this context, the technical task underlying the present invention is to provide a method for the devulcanization of sulphur vulcanized elastomers, comprising the following steps:

[0050] A. preparing an aqueous mixture by mixing vulcanized elastomers and a devulcanizing composition obtained by: i) mixing urea and at least one dicarboxylic acid in an aqueous solvent for a time comprised between 3 to 8 hours, preferably equal to 4 hours, at a temperature above 40 °C, to obtain an aqueous mixture comprising a devulcanizing ionic compound; and ii) adding a peptizing agent to said aqueous mixture coming from step i), to obtain the devulcanizing composition; and

[0051] B. carrying out a devulcanization reaction, by subjecting the aqueous mixture coming from step A to a state of stress, to obtain a devulcanized product.

[0052] Advantages of the invention

[0053] With respect to the process for preparing the modifier described in EP 1242520 Bl, which consists in simply dry mixing the at least two components of said modifier, the devulcanization method according to the present invention provides for the devulcanizer to be prepared in an aqueous solvent. This ensures better workability of the composition, not only in the preparation step, but also for subsequent uses.

[0054] Furthermore, the devulcanization method according to the present invention, thanks to mixing in aqueous solvent urea and at least one dicarboxylic acid in step A, allows to prevent crystallization phenomena of the ionic compound that is formed by interaction of the aforesaid components. In fact, their dry mixing often results in the crystallization of the salt formed by the union of the aforesaid compounds, with a consequent reduction in the efficiency of the devulcanizer, when used in the relative devulcanization processes.

[0055] Note that the devulcanization method according to the present invention allows the devulcanization of sulphur vulcanized elastomers.

[0056] In particular, compared to the known methods that provide for high operating temperatures to be used, the devulcanization method of the invention allows a better selectivity on the breaking of the crosslinks of the vulcanized elastomers, maximizing the preservation of the bonds of the main chain (C-C), as well as preventing the chains released by crosslinking with sulphur from recombining with each other, thus ensuring a greater shelf life. Advantageously, compared to the known methods employing solid-phase devulcanizers, the devulcanization method according to the present invention allows to ensure better physical and mechanical properties, in terms of tensile strength and percentage elongation at break, on the devulcanized product.

[0057] With respect to the method disclosed in patent EP1242520 Bl, which provides for a dry devulcanization, the method in accordance with the present invention provides that this reaction is carried out using a devulcanizing composition prepared in an aqueous solvent and comprising a peptizing agent.

[0058] This advantageously allows to obtain a devulcanized product having a lower viscosity and which is consequently more easily workable. In particular, the peptizing agent is used with the aim of preventing the molecular chains of the elastomers, especially of natural rubber and some synthetic rubbers, which are broken during processing to facilitate the absorption of the additives present therein, from recombining, maintaining the viscosity of the obtained mixture within values comparable to those of the masterbatch, i.e. the unvulcanized mixture. This contributes to obtaining a devulcanized product that, similar to the masterbatch, is easily dispersible in the mixture for subsequent uses.

[0059] Furthermore, advantageously, the devulcanized product obtained can be treated in a special mixer to be transformed into flakes, so as to be even more easily workable and, in particular, more easily dispersible in the mixture.

[0060] Still advantageously, the devulcanization method of the invention leads to the formation of a devulcanized product, which can be employed in combination with the unvulcanized masterbatch. The mixtures thus obtained can be subjected to vulcanization processes again and, advantageously, the resulting products (or secondary vulcanizates) have rheological, chemical-physical and / or mechanical properties, such as, for example, tensile strength and / or percentage elongation at break, comparable to those of the masterbatch.

[0061] As a result, by virtue of the aforesaid properties, the secondary vulcanizates can be advantageously exploited for further applications in the rubber field, thus ensuring continuous recycling of the raw material. DETAILED DESCRIPTION OF THE INVENTION

[0062] Definitions

[0063] By “vulcanized elastomers” is meant elastomers crosslinked with each other by formation of bivalent bonds with sulphur.

[0064] By “devulcanization” is meant a process of selective breaking of the aforesaid divalent bonds with sulphur, which allows the return of solid elastomeric objects, usually granules, into a workable and mouldable form, ready to be reused in the manufacture of new products, such as for example tyres or gaskets. In other words, by means of the devulcanization process, the vulcanized elastomers are transformed again into plastomeric, reprocessable, and / or recyclable materials.

[0065] By “devulcanizing composition” is meant a composition suitable for use in devulcanization processes.

[0066] Method for the devulcanization of sulphur vulcanized elastomers.

[0067] A method for the devulcanization of sulphur vulcanized elastomers is claimed, comprising the steps described below.

[0068] A. preparing an aqueous mixture

[0069] In a step A of the devulcanization method according to the present invention, an aqueous mixture is prepared by mixing vulcanized elastomers and a devulcanizing composition obtained by: i) mixing urea and at least one dicarboxylic acid in an aqueous solvent

[0070] In a step i) of the devulcanization method in accordance with the present invention, urea and at least one dicarboxylic acid, preferably a dicarboxylic acid, are mixed in an aqueous solvent for a time comprised between 3 and 8 hours, preferably equal to 4 hours, at a temperature above 40 °C, preferably comprised between 40 and 67 °C, to obtain an aqueous mixture comprising a devulcanizing ionic compound. Preferably, step i) is carried out in a dedicated first mixer. Preferably, said first mixer has a capacity comprised between 400 and 600 L, preferably equal to 500 L. Furthermore, said first mixer is preferably equipped with a special motor having a power comprised between 5 and 6 kW, preferably equal to 5.5 kW.

[0071] It should be noted that the mixing of urea and of the at least one dicarboxylic acid in step i) leads to the interaction between these components and, with it, to the formation of an ionic compound (or salt), formed by an organic cation, which is responsible for the devulcanizing properties of the compound.

[0072] For further details regarding the nature and mechanism of formation of the aforesaid ionic compound, reference is made to the description of patent EP1242520 Bl, which is incorporated herein by reference.

[0073] Preferably, the at least one dicarboxylic acid is a dicarboxylic acid having a carbon number of the main chain comprised between 2 and 8, preferably comprised between 2 and 6, preferably comprised between 2 and 4, preferably equal to 2.

[0074] Still preferably, the at least one dicarboxylic acid is selected from: oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, or combinations thereof, preferably it is oxalic acid.

[0075] According to a preferred embodiment, urea and the at least one dicarboxylic acid are mixed in step i) in a molar ratio comprised between 1.5: 1 and 2.5: 1, preferably equal to 2: 1.

[0076] Preferably, the at least one dicarboxylic acid is mixed in step i) in a concentration comprised between 2 and 3 M, preferably equal to 2.38 M.

[0077] Still preferably, the urea is mixed in step i) in a concentration comprised between 6.5 and 7.5 M, preferably equal to 7.1 M. ii) adding a peptizing agent.

[0078] In a step ii) of the devulcanization method according to the present invention a peptizing agent is added to the aqueous mixture coming from step i), to obtain the devulcanizing composition. Preferably, the peptizing agent comprises salts, preferably zinc salts, of aliphatic acids, preferably having a main chain length of at least 6 carbon atoms, preferably comprised between 6 and 10 carbon atoms.

[0079] In accordance with a preferred embodiment, the peptizing agent is mixed in step ii) in a quantity comprised between 0.1 and 0.7%, preferably equal to 0.5% by weight on the total weight of the devulcanizing composition (w / w).

[0080] It should be noted that preferably the aqueous mixture prepared in step A is in the form of a processable and / or mouldable aqueous slurry.

[0081] According to a preferred embodiment, the devulcanizing composition is mixed in step A in a quantity comprised between 1.0 and 4.0%, preferably equal to 3.0% by weight on the total weight of the vulcanized elastomers (w / w).

[0082] The vulcanized elastomers are preferably used in the preparation of the aqueous mixture in step A in the form of a powder or granulate, i.e. in the form of solid particles, preferably having an average diameter comprised between 0.3 and 1 mm, preferably equal to 0.8 mm.

[0083] Still preferably, the elastomers used as the raw material for obtaining the vulcanized elastomers are selected from natural rubber (NR), synthetic rubber, or mixtures thereof.

[0084] Examples of synthetic rubbers suitable for the purposes of the present invention are SBR rubber (styrene butadiene copolymer), SBR 1502 rubber, nitrile rubber (NBR), HR rubber (isoprene isobutene rubber or butyl rubber), IR rubber (synthetic polyisoprene), BR rubber (polybutadiene), SBR rubber (styrene butadiene copolymer), EPM rubber (ethylene propylene copolymer), CR rubber (polychloroprene), NBR rubber (acrylonitrile butadiene copolymer), HNBR rubber (hydrogenated nitrile), ACM rubber (ethylene acrylate copolymer), or combinations thereof.

[0085] It should be noted that preferably the elastomers used as raw material for obtaining the vulcanized elastomers comprise additives known to the person skilled in the art, preferably chosen as a function of the characteristics desired for the product to be obtained. Said additives are preferably chosen from: crosslinking agents, such as sulphur and / or organic sulphur derivatives; accelerants, capable of increasing the speed of crosslinking with sulphur (vulcanization), such as for example thiazoles, CBS (N-cyclohexyl-2- benzothiazole sulfonamide), DPG (1,3 -diphenylguanidine), TMTM, carbonzinc OZA, TBzTD, or combinations thereof; activators, capable of acting as primary accelerator activators, such as amines or zinc oxide (ZnO)-fatty acid mixtures, preferably ZnO-stearic acid mixtures; anti-caking agents, such as, for example, silica; fillers, used as inert fillers or in order to improve the properties of the elastomer, such as for example clay, calcium carbonate, and / or talc; reinforcers, such as for example carbon black; plasticizers, such as, for example, waxes, petrolatum, paraffinic oils; and stabilizers.

[0086] B. carrying out a devulcanization reaction

[0087] In a step B of the devulcanization method according to the present invention, a devulcanization reaction is carried out, by subjecting the aqueous mixture coming from step A to a state of stress, to obtain a devulcanized product.

[0088] It should be noted that preferably the devulcanization reaction occurs due to the action of the organic cations of the ionic compound of the devulcanizing composition on the vulcanized elastomers, when subjected to the state of stress, i.e. to a stressed polymeric structure state.

[0089] Advantageously, under such stress conditions, the organic cations are able to selectively break the crosslinking crosslinks between the chains of the vulcanized elastomers, maximizing the retention of the main chain bonds, i.e. the carbon-carbon (C-C) bonds.

[0090] For further details regarding the interaction scheme between the salt organic cations and the vulcanized elastomers, reference is made to the description of patent EP1242520 Bl, which is incorporated herein by reference. Preferably, in step B the vulcanized elastomers are brought to a high state of stress by transmission of the aqueous mixture through a narrow passage between the smooth rolls of a two-roll mill. Note that said narrow passage provides for a spacing between the two rolls preferably less than 0.2 mm, preferably equal to 0.1 mm.

[0091] Preferably, the transmission of the aqueous mixture through a narrow passage between the smooth rolls of a two-roll mill in step B is carried out cyclically, preferably for a number of passes of the aqueous mixture through said narrow passage comprised between 18 and 22, preferably equal to 20.

[0092] Still preferably, the rolls are governed by an inverter, which allows the rotation speed to be modified, according to procedures known to the person skilled in the art.

[0093] It should be noted that the two rolls of the mill are preferably rotated at different speeds with respect to each other, preferably in opposite directions of rotation, such as to generate a friction coefficient, calculated according to the ASTM D 1894 standard, of less than 1.8, preferably comprised between 1.2 and 1.6, preferably equal to 1.4.

[0094] Advantageously, friction coefficient values within the aforesaid ranges allow to avoid breaking the polymer chains (main chain) of the elastomers.

[0095] Preferably, each of the rolls has a diameter equal to at least 400 mm, preferably comprised between 450 and 600 mm. Still preferably, each of the rolls has a length comprised between 600 mm and 1500 mm, preferably equal to 800 mm.

[0096] Preferably, the rolls are coated with steel, preferably having a Vickers hardness (Hv) greater than 40 GPa.

[0097] In accordance with a preferred embodiment, the method for devulcanization comprises a further step C of treating the devulcanized product in a special mixer, configured to chop the devulcanized product into flakes.

[0098] It should be noted that preferably said second mixer has characteristics analogous to those of the first mixer.

[0099] Advantageously, the devulcanized product into flakes, in addition to being more easily dispersible in the mixture, can be easily packaged for subsequent use. EXAMPLES

[0100] EXAMPLE 1 : “vulcanization process of an elastomeric compound based on SBR 1502 rubber”

[0101] 5 kg of a SBR 1502 rubber-based masterbatch are prepared. Additives known and commonly used in the state of the art are added to this mixture.

[0102] The masterbatch is worked in a closed mixer, and 3 kg of the masterbatch are vulcanized in the mould, at a temperature equal to 167 °C, for a time equal to 15 minutes. The vulcanized mixture obtained is left to stand for 24 hours, to then be reduced to a powder with an average particle size of less than 800 microns.

[0103] EXAMPLE 2: “process for preparing the devulcanizing composition”

[0104] In a suitable mixer, 75 kg of oxalic acid are dissolved in 350 L of water, at a temperature at least equal to 40 °C, preferably comprised between 40 °C and 67 °C. The system is kept under stirring, for a time at least equal to about 5 minutes, until the oxalic acid is completely dissolved in water. 150 kg of urea are added to the solution of acid in water, and the system is kept under stirring for a time comprised between 4 and 8 hours, until the reaction of formation of the ionic compound (or salt) between urea and oxalic acid is ended.

[0105] Upon completion of the reaction, a peptizer based on a mixture of zinc soaps of high molecular weight fatty acids is added, in a quantity equal to 0.5% by weight on the weight of the devulcanizing composition thus obtained (w / w).

[0106] EXAMPLE 3 : “devulcanization of the vulcanized SBR rubber-based compound”

[0107] In a special mixer, 3 kg of vulcanized mixture, in powder form, prepared according to the procedure described in EXAMPLE 1, are mixed with the devulcanizing composition obtained according to the procedure described in EXAMPLE 2, in a quantity equal to 3% by weight on the weight of the vulcanized mixture (w / w).

[0108] The mixture thus obtained is subjected to a state of stress, by passing it within a narrow passage between two smooth rolls of a two-roll mill. This operation is carried out cyclically, for a total number of passes equal to 20, at a recorded temperature equal to 38 °C. The devulcanized product thus obtained is treated in a special mixer, so as to make it in flakes.

[0109] EXAMPLE 4: “characterization of a vulcanized compound comprising the SBR rubber-based masterbatch and the devulcanized product” A mixture is prepared by mixing in a closed mixer 2 kg of unvulcanized masterbatch used in EXAMPLE 1 with the devulcanized product obtained according to the procedure described in EXAMPLE 3, in flakes, in a ratio by weight of devulcanized product to elastomers of the masterbatch (thus excluding additives) equal to about 30:70. The mixing is carried out for a time equal to 15 minutes. After mixing, the mixture obtained is subjected to further vulcanization, following the same vulcanization procedure described in EXAMPLE 1, obtaining a final vulcanized product (or secondary vulcanizate).

[0110] Experimental tests of chemi cal -physical and mechanical characterization are carried out on the secondary vulcanizate, the results of which are summarized below in Table 1. between the SBR secondary rubber vulcanizate and the masterbatch”

[0111] From the experimental data reported in Table 1, a significant improvement in both tear strength and elongation at break values can be seen. EXAMPLE 5: “characterization of a vulcanized mixture obtained from a mixture of masterbatch based on butyl rubber and devulcanized product”

[0112] Following the same procedures of vulcanization, preparation of the devulcanizing composition, devulcanization, and re-vulcanization, respectively described in EXAMPLES 1-4, but using a compound based on butyl rubber (HR rubber) as the masterbatch, experimental tests of rheological, chemical-physical and mechanical characterization are carried out on the final vulcanized (or secondary vulcanized) product. The results of these tests are summarized below in Table 3.

[0113] The composition of the masterbatch and of the final vulcanized product are reported below in Table 2.

[0114] Table 2: “masterbatch composition based on butyl rubber vs secondary vulcanizate”

[0115] Note from Table 2 that, excluding the additives, out of 1.800 Kg of elastomer (butyl HR) of the masterbatch, 0.400 Kg of devulcanized product, obtained from 0.400 Kg of elastomer of the masterbatch, and 1.400 Kg of the remaining elastomer deriving from the masterbatch, i.e. a weight ratio between devulcanized product and elastomer of the masterbatch equal to about 29:71, were used in the secondary vulcanizate. The quantity of each additive between the masterbatch and the secondary vulcanizate remained substantially unchanged. between the butyl rubber secondary vulcanized product and the masterbatch” From the data reported in Table 3, an improvement in tensile strength at break can be observed, while there are no significant changes, which cannot be corrected by appropriately choosing additives known to the person skilled in the art, of the other rheological / mechanical properties of the secondary vulcanizate compared to the masterbatch. EXAMPLE 6: “characterization of a vulcanized compound obtained from masterbatch based on hard NBR rubber and a devulcanized product”

[0116] Following the same procedures of vulcanization, preparation of the devulcanizing composition, devulcanization, and re-vulcanization, respectively described in EXAMPLES 1-4, but using a hard NBR rubber-based compound as the masterbatch, experimental rheological, chemi cal -physical and mechanical characterization tests are carried out on the final vulcanized product (or secondary vulcanizate). The results of these tests are summarized below in Table 5.

[0117] The composition of the masterbatch and the final vulcanized product (secondary vulcanizate) are reported below in Table 4. Table 4: “masterbatch composition based on NBR rubber vs secondary vulcanizate”

[0118] Note from Table 4 that, excluding the additives, out of 1.800 Kg of elastomer (NBR rubber) of the masterbatch, 0.400 Kg of devulcanized product, obtained from 0.400 Kg of elastomer of the masterbatch, and 1.400 Kg of the remaining elastomer from the masterbatch, i.e. a weight ratio between devulcanized product and elastomer of the masterbatch equal to about 29:71, were used in the secondary vulcanizate. The quantity of each additive between the masterbatch and the secondary vulcanizate remained substantially unchanged. between the secondary vulcanizate from hard NBR rubber and the masterbatch”

[0119] From the data reported in Table 5, it can be observed that there are no significant changes, which cannot be corrected by appropriately choosing additives known to the person skilled in the art, of the other rheological / mechanical properties of the secondary vulcanizate compared to the masterbatch.

[0120] EXAMPLE 7: “characterization of a vulcanized compound obtained from a hard SBR rubber-based masterbatch and a devulcanized product”

[0121] Following the same procedures of vulcanization, preparation of the devulcanizing composition, devulcanization, and re-vulcanization, respectively described in EXAMPLES 1-4, but using a compound based on natural rubber and SBR 1502 rubber as the masterbatch, experimental tests of rheological, chemi cal -physical and mechanical characterization are carried out on the final vulcanized (or secondary vulcanized) product. The results of these tests are summarized below in Table 7. The composition of the masterbatch, as well as the final vulcanized product (secondary vulcanizate), are reported below in Table 6. secondary vulcanized”

[0122] Note from Table 6 that, excluding additives, out of 1.800 Kg of elastomers (natural rubber + SBR 1502) of the masterbatch, 0.400 Kg of devulcanized product was used in the secondary vulcanizate, obtained from 0.400 Kg of elastomer (0.100 Kg of natural rubber + 0.300 Kg of SBR 1502) of the masterbatch, and 1.500 Kg (0.900 Kg of natural rubber + 0.600 Kg of SBR 1502) of the remaining elastomers from the masterbatch, i.e. a weight ratio between devulcanized product and elastomers of the masterbatch equal to about 27:73. The quantity of each additive between the masterbatch and the secondary vulcanizate remained substantially unchanged.

[0123] Table 7: “comparison of the rheological, chemi cal -physical and mechanical properties between the secondary vulcanizate of natural rubber and hard SBR rubber and the masterbatch”

[0124] Again, from the data reported in Table 7, it can be observed that there are no significant changes, which cannot be corrected by appropriately choosing additives known to the person skilled in the art, of the other rheological / mechanical properties of the secondary vulcanizate compared to the masterbatch.

Claims

CLAIMS1. Method for the devulcanization of sulphur vulcanized elastomers, comprising the following steps:A. preparing an aqueous mixture by mixing sulphur vulcanized elastomers and a devulcanizing composition obtained by: i) mixing urea and at least one dicarboxylic acid in an aqueous solvent, for a time comprised between 3 and 8 hours, preferably equal to 4 hours, at a temperature above 40°C, to obtain an aqueous mixture comprising a devulcanizing ionic compound; and ii) adding a peptizing agent to said aqueous mixture coming from step i), to obtain the devulcanizing composition.B. carrying out a devulcanization reaction, by subjecting the aqueous mixture coming from step A to a state of stress, to obtain a devulcanized product.

2. Method according to claim 1, wherein step B is carried out by passing the aqueous mixture in a two-roll mill, said step B being carried out cyclically, preferably for a number of passes of the aqueous mixture comprised between 18 and 22.

3. Method according to claim 1 or 2, wherein in step A the devulcanizing composition is mixed in a quantity comprised between 1.0 and 4.0% by weight on the total weight of the vulcanized elastomers (w / w).

4. Method according to any one of claims 1 to 3, comprising a further step C of treating the devulcanized product in a special mixer, configured to chop the devulcanized product into flakes.

5. Method according to any one of claims 1 to 4, wherein urea and the at least one dicarboxylic acid are mixed in step i) in a molar ratio comprised between 1.5: 1 and 2.5: 1, preferably equal to 2: 1.

6. Method according to any one of claims 1 to 5, wherein the at least one dicarboxylic acid is selected from: oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, or combinations thereof, preferably it is oxalic acid.

7. Method according to any one of claims 1 to 6, wherein the peptizing agent comprises salts, preferably zinc salts, of aliphatic acids having a main chain length of at least 6 carbon atoms.

8. Method according to any one of claims 1 to 7, wherein the peptizing agent is added in step ii) in a quantity comprised between 0.1 and 0.7% by weight on the total weight of the devulcanizing composition (w / w).

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